It is important to provide temperature sensors with high response times in nuclear reactor applications and, in particular, for measuring the temperature at upstream and downstream sides of the nuclear reactor core in a coolant loop of a pressurized water nuclear reactor.
A pressurized water nuclear reactor (PWR) generally has a coolant loop or circulation path which includes a pump for displacing the water coolant under pressure through the nuclear reactor core, an expansion chamber or accumulator for taking up pressure surges, and as a heat exchanger, a boiler or vapor generator which is heated by the coolant at elevated temperatures derived from the PWR.
In a system of that type, a temperature sensor can be provided to effect three principal functions:
1. Measurement of the power generated.
The measurement of the power generated can be effected by application of a law of the type: EQU P=Q.sub.1 -Q.sub.2
with EQU Q.sub.(1,2) =MCT.sub.(1,2)
in which:
Q is the quantity of heat supplied per unit of time and Q.sub.1 is the heat per unit of time at the inlet of the exchanger while the heat flow at the outlet of the heat exchanger is represented at Q.sub.2 ; PA0 M is the mass flow of the heat carrying fluid per unit of time; PA0 C is the specific heat of the fluid at the temperature under consideration; and PA0 T is the absolute temperature of the fluid.
If the quantities of the fluid entering and leaving the exchanger are equal: EQU P=MC.DELTA.T
where .DELTA.T=T.sub.1 -T.sub.2.
The precision with which the power can be measured thus required precise knowledge of the quantity of water circulated in the loop, the absolute temperature in order to determine with precision the specific heat, and the difference between the inlet and outlet temperature of the exchanger or the vapor or steam generator.
The energy produced by the nuclear reactor can only be withdrawn in a useable manner under thermodynamicokinetic conditions in the core of the reactor which can be generalized as requiring a high volumetric flow rate and a low thermal gradient.
Present instruments permit determination of flow rates with the precision of about 1% and determination of temperature with a precision of about 0.1.degree. C.
2. The safety of a PWR reactor is assured utilizing various sensors disposed throughout the plant perhaps the most important of them are temperature sensors which provide an indication of the instantaneous temperature prevailing at critical points in the cooling loop. As a consequence, the speed with which the temperature sensor responds to a change in temperature is basic to avoidance of incidents or accidents and to permit personnel to take immediate action to safeguard the reactor and its environment.
Present standards require that the thermometric sensor be capable within a maximum of four seconds, of providing a temperature which represents 63% the true value of a changed temperature to which the sensor is subject, i.e. 63% of the true temperature or temperature variation.
3. It is essential, considering the lowered exposure levels now deemed to be permissible for nuclear power plant operating personnel to provide the sensors so that they may be readily replaced in a minimum of time and with a minimum of complexity.
In the past, as the discussion below will show in greater detail, it has been deemed to be advantageous to provide the temperature sensors in bypasses of the pressurized water loop, i.e. in additional piping which is separate from the traversed by the main pressurized water flow. This system had the disadvantage of complicated mounting and dismounting, thereby requiring prolonged presence of the plant personnel at the high radiation sites. Because the temperature was taken in a bypass, moreover, the response time to the actual temperature in the cooling loop was low. This system could, however, utilize a sensor which made use of the change in resistivity of a platinum wire.
Another arrangement which was used provided a tubular finger or closed-end sheath which could project into the path of the liquid and into which a sensor was inserted. The adjustment of the sensor within the sheath was complex, irregular clearances were defined and the response time was poor because of the clearance required between the sensing end of the sensor and the sheath.
The system here was also susceptible to significant vibration which caused rapid deterioration of the sensor.